The invention relates to a light source for optical data transmission and to a fibre amplifier for amplifying an optical input signal.
Optical fibres offer an enormous capacity for optical data transmission in binary form. Most such systems currently in operation work with lasers which emit light with a wavelength of approximately 1300 nm. The laser light is modulated with the data to be transmitted and is coupled into monomode fibres which transport the data over the desired distance. Known techniques for light sources, modulators, detectors, amplifiers etc. permit a transmission rate of 10 Gbps with this technique.
In order further to increase the transmission rates or capacities, one single optical fibre is used for simultaneous transmission of a plurality of data channels. In this case light with a different wavelength is used for each individual data channel. This technique is known as o wavelength domain multiplexing (WDM). In this case each individual data channel retains a transmission capacity of 10 Gbps. The total transmission capacity of the system is then a multiple of 10 Gbps.
In order to achieve WDM the largest possible plurality of wavelengths is required simultaneously. For this purpose at present a corresponding plurality of usually monochromatic lasers, e.g. semiconductor lasers, is operated simultaneously alongside one another. In this case each laser requires its own control system and its own power supply unit. Overall a plurality of individual lasers increases the susceptibility of the total system to disruption and increases the costs.
The object of the invention is to create a single simple and economical light source or such a fibre amplifier for the optical communication by means of WDM.
This object is achieved according to the invention by a light source with the features of claims 1 or 16 or respectively by a fibre amplifier with the features of claim 12.
The light source according to the invention has a pumping light source for generating pumping light at a first wavelength. This pumping light source may be a suitable lamp. As a rule, however, the pumping light source will be a laser, preferably a semiconductor laser. The central element of the light source is an optical fibre which is doped with an active laser medium. Rare-earth ions such as erbium or neodymium may be considered in particular as active laser medium. The pumping light source must be chosen in such a way that the laser medium can be excited at the wavelength of the pumping light source.
Most laser media, particularly the rare-earth ions, generate emission light with a wavelength spectrum with at least one wavelength range of high power, typically two maximum values, and at least one wavelength range with relatively lower power, typically a minimum value between the maximum values.
In order that the laser medium can be excited, the light source has a means for coupling the pumping light into the doped optical fibre. This means may consist of concave mirrors which guide the light laterally onto the doped optical fibre, or it may be an optical arrangement for final pumping of the doped optical fibre.
The active laser medium absorbs a proportion of the pumping light and as a result it goes over into an electronically excited state. Then the active laser medium spontaneously emits wideband radiation and does so to some extent at such shallow angles that some of the radiation is captured in the doped optical fibres. The captured spontaneous radiation is amplified in the doped optical fibre by stimulated emissions (amplified spontaneous emission ASE).
Most laser media have an emission spectrum which is characterised by maximum power values and minimum power values, that is to say it is far from being uniform. According to the invention light in the wavelength range with relatively lower power of the emission spectrum is partially selected with the aid of a selection means in or on the doped optical fibre. In this way this part of the spectrum is preferably amplified during further passage through the doped optical fibre.
In a total system for optical data transfer the wide spectrum is then broken down into individual wavelengths. This may take place in various known manners, for instance with narrow-band interference filters connected downstream or with the aid of gratings, e.g. in a so-called demultiplexer. The individual wavelengths are in each case modulated by themselves in order to transmit data and thus each form a transmission channel. Then the various wavelengths are brought together again, for instance in a so-called multiplexer, and are jointly coupled into an optical fibre and transmitted through it.
The wideband light source according to the invention is distinguished by an extremely simple construction. Therefore it is less susceptible to disruption, it is robust and can be produced at low cost.
At least one Fabry-Pxc3xa9rot filter or an interference filter can be inserted as selection means into a portion of the doped optical fibre. Furthermore a periodic modulation of the refractive index in the core of the doped optical fibre can be provided.
In an advantageous embodiment of the invention the doped optical fibre is a monomode fibre. The wideband light is generated thereby with minimal divergence. Furthermore it can be easily coupled into other fibres.
A particular convenient construction of the wideband light source according to the invention is produced if the pumping light source is a semiconductor laser, the radiation of which is coupled into a second monomode fibre. Furthermore an optical coupler which has three ports can be used, the first port being coupled to the second monomode fibre, the second port being coupled to one end of the doped optical fibre and the third port being coupled to a third monomode fibre. In this case the coupler is constructed in such a way that the pumping light from the second monomode fibre is coupled into the doped optical fibre. Thus the doped optical fibre is finally pumped. Furthermore the coupler is constructed in such a way that the emission light from the doped optical fibre is coupled into the third monomode fibre. Thus the emission light proceeds without power losses to the output of the wideband light source. This construction is also particularly well suited to mass production and assembly.
In order to obtain an even wider spectrum, the doped optical fibre can be doped with a mixture of various rare-earth ions.
In an advantageous embodiment the light source according to the invention additionally has a means for selecting a plurality of individual wavelength ranges from the emission spectrum. This means may be a Fabry-Pxc3xa9rot filter or a demultiplexer. Or it may be formed by a periodic modulation of the refractive index in the core of the doped optical fibre (Bragg grating), the periodic modulation being designed in such a way that it reflects a plurality of individual wavelength ranges.
By such an additional means the total radiant energy is concentrated onto the lines or spectral ranges which are really used. Thus the radiant energy is better utilised.
By the use of a Fabry-Pxc3xa9rot filter as the said additional means the line width as well as the spacing or the number of lines can be freely chosen and altered. Thus a system which can be reconfigured is obtained which can be reconfigured in a simple manner. Also the wavelength of the lines can be regulated, e.g. by comparison with a reference wavelength.
Thus overall a simple light source is obtained which generates individual lines at different wavelengths with the same intensity.
The inventive ideas for the light source according to the invention can also be advantageously used for a fibre amplifier for amplifying an optical input signal. Such a fibre amplifier has a pumping light source for generating pumping light at a first wavelength. It also has an optical fibre which is doped with an active laser medium which can be excited at a first wavelength, whereby it can emit emission light with a wavelength spectrum with at least one wavelength range of high power and at least one wavelength range with relatively lower power. The pumping light source and the optical input signal are coupled by way of a coupler into a first end of the doped optical fibre.
A selection means is located in the doped optical fibre, usually immediately after the coupler. It selects light in the wavelength range with relatively lower power of the emission spectrum, whereby this part of the spectrum is preferably amplified during further passage through the doped optical fibre.
At the second end of the doped optical fibre is located an optical isolator which allows the passage of light which enters the optical fibre from the first end.
The fibre amplifier according to the invention offers a smooth amplification profile for the majority of wavelengths which in principle can be amplified with the doped fibre.
A particularly simple construction of the amplifier is produced if a suitable adjusted Fabry-Pxc3xa9rot filter is inserted as selection means into a portion of the doped optical fibre.
Similar effects may be achieved with an interference filter which moreover has the advantage that it leads to a monolithically compact construction.
Furthermore a periodic modulation of the refractive index may be provided as selection means in the core of the doped optical fibre.
A further light source according to the invention has a plurality of wideband light sources which emit light with a flat spectral development in each case in the respective spectral range, wherein the spectral ranges of the wideband light sources overlap and are slightly displaced relative to one another. The wideband light sources are preferably light emitting diodes (LED). However, they may also be light sources as claimed in claim 1. The light from these wideband light sources is initially overlaid. Then a plurality of individual wavelength ranges is selected from the overlaid spectrum, for example with the aid of a Fabry-Pxc3xa9rot filter. In this way a light source is obtained which emits a plurality of individual spectral lines.
Further advantageous embodiments of the inventions are characterised in the subordinate claims.